1. Field of the Invention
The present invention relates to a tandem-type image formation system including two electrophotographic image forming apparatuses connected together in tandem, in which an image is formed on a first surface (front surface or rear surface) of a sheet in the image forming apparatus disposed on the upstream side in the sheet conveyance direction, and an image is formed on a second surface (rear surface or front surface) of the sheet in the image forming apparatus disposed on the downstream side. The present invention also relates to an density controlling method in the image formation system.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a uniformly-charged photoconductor (for example, a photoconductor drum) with (to) light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet through an intermediate transfer belt, followed by heating and pressurization for fixing, whereby an image is formed on the sheet.
In such an electrophotographic image forming apparatus, an image density control (also called “image stabilization control”) is performed to stably maintain desired image quality. To be more specific, a toner pattern for density detection is formed on an image bearing member such as an intermediate transfer belt, and the density of the toner pattern (toner adhesion amount) is detected using a light sensor of a reflection type or a transmission type. Then, the image formation conditions such as the light exposure energy (light exposure time or light exposure output), charging voltage, developing bias voltage, rotational frequency of a developing roller are controlled based on the detection results such that the density of the toner pattern matches the target control density. The target control density is set in accordance with the level of density control points which are set in a stepwise manner. The light sensor used in the image density control is called “IDC (Image Density Control) sensor.”
Furthermore, techniques have been proposed in which the image formation condition is controlled in accordance with the tendency of the coverage rate (coverage) in the case where image formation is continuously performed on a plurality of sheets (PTL 1: Japanese Patent Publication No. 4995123 and PTL 2: Japanese Patent Publication No. 4949672).
In recent years, in the field of the production print in which a high throughput is demanded, the tandem-type image formation system in which two electrophotographic image forming apparatuses in tandem are connected with each other is put in practical use. In general, in a tandem-type image formation system, a sheet inversion apparatus is disposed between an image forming apparatus on the upstream side in the sheet conveyance direction (hereinafter referred to as “upstream apparatus”) and an image forming apparatus on the downstream side (hereinafter referred to as “downstream apparatus”). When duplex image formation is performed, first, an image is formed on a first surface of a sheet (front surface or rear surface) by the upstream apparatus, and then the sheet is inverted by the sheet inversion apparatus, and thereafter, an image is formed on a second surface of the sheet (rear surface or front surface) by the downstream apparatus.
Also in the above-described tandem-type image formation system, the image density control is performed in each of the two image forming apparatuses. However, the output performances of the IDC sensors of the image forming apparatuses may be different from each other. Therefore, even when the image density control of the same density control point is performed, the densities of the images actually formed on the sheet may not be the same. This may results in a difference in the density between the front and rear surfaces of the sheet, and a difference in the image quality between the front and rear surfaces.
In addition, in the case where two-component developer is used, a difference in the consumption amount of toner, or in other words, a difference in the amount of toner to be supplied, is caused due to a difference in the coverage rate of the original image data even when the density control point of the IDC sensor is corrected such that the image densities on a sheet are identical to each other between the upstream apparatus and the downstream apparatus at an initial stage of the image formation. In this manner, the development performance is changed, and consequently, a density difference between the front and rear surfaces may possibly be gradually caused.